The present invention relates to hydrokinetic turbines designed for the purpose of generating electricity, and to methods for designing and using such turbines. It further relates to certain elements employed in hydrokinetic turbines. The turbines according to the invention are intended to be placed underwater, in a fixed, floating, anchored or towed configuration, in any location where the effective water current preferably flows with a minimum speed of about 0.25 m/s. The water flow or current may be of any type or source, although typically it is comprised of one or more of the following types of water flow or current:                a) Fixed, floating or anchored in continuous water flow or current, as found, e.g., in ocean currents, rivers or streams.        b) Fixed, floating or anchored in fluctuating, alternating and/or cyclical water flow or currents that may change direction periodically or irregularly, as found, e.g., in tidal flow or seasonal flow.        c) Fixed, floating or anchored in mechanically or naturally induced occurring currents that are created by, e.g., filling and emptying of reservoirs, lakes, dams or locks.        d) The device may be towed through the water by a vessel or other device or method to artificially or effectively create a flow through the device.        
The power of flowing water has been used by mankind for millennia to generate energy of various kinds for many different purposes. It has been used for milling grain, belt driven applications to run machines in factories and to power many kinds of devices mechanically. For the last 150 years water flow has proven to be very efficient for electrical generation in countless different designs and applications.
The basic principle of using permanent magnets and copper coils to generate electricity is still being used today in many different forms, including using flowing water and water turbines to drive electrical generators and alternators.
Most ocean currents are caused by wind, which in turn is caused by the Coriolis forces coming from the rotation of the earth. These currents are often influenced by the position of landmasses that can divert the flow and in some cases accelerate the flow. Ocean currents can also be caused by density differences in water masses, temperature differences or variations in the salinity of the water. The ocean currents on this planet are probably the biggest untapped source of energy in existence. River currents are also often used as a very good and efficient source of energy.
Since the beginning of technological development, there have been many different attempts made to harvest this energy with varying degrees of success and efficiency. The currents that are most accessible and easiest to use for energy generation are near-shore surface currents of the ocean and river currents. Water flow can also be produced artificially by building dams and creating reservoirs to accumulate large masses of water that can be utilized on demand.
In 1882 the world's first hydroelectric power plant was on the Fox River in Appleton Wis. By 1889, 200 electrical plants were built in the USA, and by 1920, hydropower was used for 25% of US electrical generation, which usage by 1940 went up to 40%. Today only 6 to 8% of the electricity produced in the United States comes from hydropower. There are vast opportunities and significant environmental and cost advantages to be gained by replacing conventional coal-fired power plants with hydroelectric installations. Older installations of hydroelectric power plants are mostly situated inside dams or below dams using the pressure at the bottom of the dam to operate a water turbine that drives electric generators.
Since World War I the field of science, today called fluid dynamics, has developed tremendously and become a very precise and finite science which is used today in the design of modern hydrofoils. Hydrofoils (as well as airfoils, also part of fluid dynamics) are used for a large variety of purposes, including most designs in aeronautics, in motor vehicles, in watercraft, and in isolated elements employed in hydrokinetic turbines.
Hydrokinetic turbines can be divided up into different categories or types. For example, a turbine can either be bi-directional or unidirectional. In the former case, the turbine in defined such that it can be operated by a current that flows in both axial directions through the turbine, e.g., to be actuated to generate power both by an incoming tidal flow as well as by a receding tidal flow. On the other hand, a unidirectional turbine is driven only by the flow of water in a single axial direction. From a hydrodynamic standpoint, the design criteria to produce a bi-directional turbine are significantly more limited than in the case of a unidirectional turbine, i.e., all design criteria that would produce an adverse effect upon reversal of fluid flow direction.
Another way of categorizing hydrokinetic turbines resides in their hub design, namely, whether the center hub is either closed or open. Traditionally, most hydrokinetic turbines possess a non-rotating (fixed with respect to the turbine outer shroud) center hub that is closed or solid, and about which the rotor blades rotate. See, e.g., the following documents for examples: U.S. Pat. No. 3,986,787 to Mouton et al., U.S. Pat. No. 4,221,538 to Wells, U.S. Pat. No. 4,313,711 to Lee, U.S. Pat. No. 4,421,990 to Heuss et al., U.S. Pat. No. 6,168,373 to Vauthier, U.S. Pat. No. 6,406,251 to Vauthier and GB 2,408,294 to Susman et al. Some designs have a solid center hub, but rotate about bearings between a radially outer rotor ring and a turbine shroud, as disclosed, e.g., in U.S. Pat. No. 4,163,904 to Skendrovic.
More recently, one company has pursued hydrodynamic turbine designs in which there is provided an open center hub, for environmental reasons, i.e., to provide a safe passageway for sea creatures. See, e.g., the following documents for examples: U.S. Pat. Nos. 6,957,947, 7,378,750, 8,308,422 and 8,466,595. In these basically hubless designs, the rotor blades are typically mounted at the radial inside upon an inner ring member, and on the radial outside on an outer ring member, and in some designs, there is no inside ring member present at all. These basically hubless turbine designs are all bi-directional and are axially symmetrical in design.
In an adaptation of the open center concept, a type of hydrokinetic turbine is disclosed that is of the fixed center hub design noted above, but also includes a passageway or an opening in the center hub. See, e.g., US 2013/00443685 to Sireli et al., U.S. Pat. No. 7,471,009 to Davis et al., both of which relate to a unidirectional turbine design. Also see, U.S. Pat. No. 7,874,788 to Stothers et al., and US 2010/0007148 to Davis et al., which relate to specially-configured, bi-directional hydrokinetic turbines that include the optional use of an open center hub or, in the latter, a bypass opening in the hub, as in related Davis et al. '009, noted above (see FIG. 7 of both).
Hydrokinetic power generation remains of great interest and has gained growing importance along with solar power and wind power. There is a need for significant effort to be made to design and build much more sophisticated and highly efficient hydrokinetic power-generating turbines; however, because the process of refining turbine designs is in many respects unpredictable and therefore time-consuming, there has unfortunately been a tendency to simply build larger versions of existing turbine designs in order to gain larger energy output from them. New, highly efficient turbines will enable the extraction of increased amounts of energy from a renewable source, with practically no environmental impact. Further improvements in such turbines are highly desired, for these reasons.